spv/lift: use loop body entry (instead of exit), as the "continue" target.

[eddyb]

As we have no break edges out of SPIR-T loop bodies, any point in the loop body (not nested in another selection/loop) is a valid "continue" target, and we can spare a block by making the whole body that.

In WGSL loops, this corresponds to moving the rest of the loop body (not using break/continue) into the continuing {...} block. _{(but examples are hard because break if isn't implemented yet in Naga, and our do-while-style loops use SPIR-V "conditional back-edges" which in WGSL require break if - see gfx-rs/naga#1977 (comment) for more details/context)}

For tests/data/for-loop.wgsl.spvasm (after spirv-as -> spv-lower-link-lift -> spirv-dis), the change is:

@@ -25,5 +25,5 @@
          %22 = OpLabel
-         %13 = OpPhi %int %int_1 %21 %16 %27
-         %14 = OpPhi %int %int_1 %21 %17 %27
-               OpLoopMerge %28 %27 None
+         %13 = OpPhi %int %int_1 %21 %16 %26
+         %14 = OpPhi %int %int_1 %21 %17 %26
+               OpLoopMerge %28 %23 None
                OpBranch %23
@@ -44,4 +44,2 @@
          %17 = OpPhi %int %20 %24 %11 %25
-               OpBranch %27
-         %27 = OpLabel
                OpBranchConditional %15 %22 %28

_{(on the "after" side I had to use sed 's/%27/%28/g' on to make the diff more readable wrt renumbering)}

[eddyb]

Will sadly have to revert this, because it fails to account for divergence in the loop body.